In the field of optical devices, such as, displays, light emitting diodes and solar cells, various proposals have been recently made for the purposes of energy conservation and of improvement in light utilization efficiency. For example, there is a known method for increasing the aperture ratio of a liquid crystal display. In that method, a transparent planarization film is formed to cover a TFT element and then pixel electrodes are formed on the planarization film (see, Patent document 1).
As materials for the planarization film formed on the TFT substrate, it is known to adopt an acrylic resin and a quinone diazide compound in combination (see, Patent documents 2 and 3). Those materials have both photosensitivity and planarization properties, and hence can form a contact hole and other patterns. However, wiring has become complicated in accordance with improvement of the resolution and frame frequency, and accordingly planarization conditions have become so severe that it has been getting difficult for the above materials to satisfy the conditions.
Meanwhile, polysiloxanes are known as a material for forming a cured film of high heat resistance, of high transparency and of high resolution. Particularly among them, silsesquioxane derivatives have been widely employed because they have low permittivity, high transparency, high heat resistance, tough UV durability, and good coating uniformity. A silsesquioxane is a polymer comprising a trifunctional siloxane structural unit RSi(O1.5), which can be considered to have an intermediate chemical structure between an inorganic silica structure (SiO2) and an organic silicone (R2SiO). This polymer is such a specific compound as is soluble in an organic solvent but forms a cured product characteristically having high heat resistance almost comparable to that of inorganic silica.
A phenyl-containing polysiloxane is known to be inexpensive and high in safety. For example, there is a known photosensitive material comprising a phenyl-containing polysiloxane, a quinone diazide compound, a solvent and a surfactant. This photosensitive material is characterized in that the polysiloxane has high solubility in the solvent and also in that a cured film formed therefrom hardly suffers from cracking. Specifically, the cured film formed from that material is excellent in high-temperature durability and in uniformity, and also has high transparency, low permittivity, good electrical insulating properties, and tough chemical resistance (see, Patent documents 4 and 5). However, the phenyl-containing polysiloxane has a problem of emitting harmful and carcinogenic volatile substances, such as benzene, in the thermal curing process.
In view of the above, it is proposed to adopt a silsesquioxane having an aryl group (which is not a phenyl group but a substituent-containing phenyl group) so as to produce a photosensitive siloxane composition which does not emit benzene (see, Patent documents 6 and 7). However, that kind of siloxane tends to cost a lot because produced by a special process. Further, it also tends to be inferior to the phenyl-containing polysiloxane in high-temperature durability and in UV durability. Furthermore, the above composition tends to undergo gelation if a photo base-generator, which is known to be used for the purpose of promoting polymerization of resins (see, Patent documents 8 and 9), is employed for polymerization of the siloxane.